Lignocellulosic materials are sources for the generation of a variety of products. Some of the products retain significant structural components of the lignocellulose such as mechanical pulp fibers from wood chips. Other compounds such as sugars derived from the carbohydrate in lignocellulose are made into products by fermentation or chemical conversion. The lignocellulose can be made into products that represent a continuum of structured to molecular products. The continuum of products is generated by a variety of physical, chemical, biological and thermal processes.
In manufacturing paper from wood, the wood is first reduced to an intermediate stage in which the fibers in the wood are separated from their natural environment and transformed into a viscous liquid suspension called pulp. One of the components of wood is lignocellulose. The most abundant component of lignocellulose are the cellulose polymers. These are the most desired polymers in the final pulp product. The second most abundant polymer, and least desirable pulp component of lignocellulose, is lignin. Lignin is undesired because substantial amounts of lignin in pulp can reduce the smoothness of the final paper product and cause the paper to discolor when exposed to light. Lignin can also cause the pulp fibers to be rigid and weak.
The third major component of lignocellulose is the hemicellulose. Hemicelluloses are polymers of sugars that are more heterogeneous than cellulose. The hemicelluloses are comprised of oligomeric sugars derived from arabinose, galactose, xylose and mannose in addition to glucose. The hemicellulose and the lignin are intermixed with the cellulose in lignocellulose and serve to protect the cellulose from damage by organisms, enzymes or chemicals. Removal of the hemicellulose and lignin is often a portion of lignocellulose processing.
Pulp may be produced from various types of lignocellulose using any one of several pulping techniques. The simplest of these techniques is the refiner mechanical pulping (RMP) method in which a mechanical milling operation grinds or abrades wood in water until a desired state of freeness (an arbitrary measure of water drainage) is achieved between its fibers. The RMP method is high yield, typically converting approximately 95% of the dry weight of the wood into pulp. The RMP method, however, also leaves substantially all of the lignin and hemicellulose in the pulp. As a result, RMP pulps generally provide low strength paper products having an opaque color. These paper products are generally used to manufacture newsprint or other low quality paper products.
Other pulping methodologies include thermo-mechanical pulping (TMP), chemical treatment with thermo-mechanical pulping (CTMP), chemi-mechanical pulping (CMP), and the chemical pulping, sulfate (kraft) or sulfite processes. In the chemical based methods, a chemical/water solution is generally used to dissolve the lignin and hemicellulose to promote the separation of the fibers. The absence of lignin, in turn, makes the final paper products stronger and less prone to discoloration. These products often include paper bags, shipping containers, printing and writing papers, and other products requiring strength.
In thermo-mechanical processes (e.g. TMP and CTMP), high temperatures are used to separate the fibers during refining. These processes generally require the refining to be carried out in one or more steps. The first step is usually a pressurized step with refining being performed at temperatures above 100° C. and immediately below or at the softening temperature of lignin. During this step, the pulp is typically mechanically processed using the RMP method. In subsequent steps, the pressure and temperature is usually modulated to achieve the desired state of freeness between the fibers.
Relatively high total electric energy amounts or high quantities of input lignocellulose are required to produce pulps using the above mentioned pulping techniques. In particular, high energy inputs are generally required to obtain fiber separation in woods rich in lignin as such woods typically call for extended refining periods and higher refining temperatures or pressures. Recent studies have also suggested that even thermal or chemical softening treatments of such woods do not guarantee a lower total energy consumption. This is because unprocessed fibers which are only mildly separated by the thermal or chemical treatments are difficult to fibrillate during the refining mechanical process.
Fibrillation is necessary to increase the flexibility of the fibers and bring about the fine material characteristics of quality processed pulp. In fact, it has been suggested that a decrease in the energy consumption from an established level in various TMP and CTMP processes has been associated with the deterioration of certain pulp properties, including a reduction in the long fiber content of the pulp, a lower tear strength and tensile strength, and a higher shives content. (See U.S. Pat. No. 5,853,534, incorporated herein by reference). As a result, high energy consumption in TMP and CTMP processes has been generally necessary in today's pulping practices.
An improved method is needed for producing pulp which is energy efficient, produces paper having improved properties, with fewer undesirable process byproducts (especially environmentally objectionable byproducts), and with an increased production of useable high end desirable products e.g. hemicellulosic sugars. A method shown to affect critical components of the lignocellulose such as the hemicellulose should be useful for pulping lignocellulose and also to prepare lignocellulose for total dissolution into sugars and lignin.